1. Field of the Invention
The present invention relates to integrated circuits. More particularly, it relates to a method for producing nanoporous dielectric coatings useful in the production of integrated circuits.
2. Description of the Prior Art
In the production of integrated circuits, the problems of interconnect RC delay, power consumption and crosstalk become more significant as feature sizes approach 0.25 .mu.m and below. It has been found that the use of low dielectric constant (K) materials for interlevel dielectric and intermetal dielectric applications partially mitigate these problems. However, each of the material candidates which are under consideration by the industry, having dielectric constants significantly lower than the currently employed dense silica, suffer from disadvantages. Most low dielectric constant materials developments use spin-on-glasses and fluorinated plasma chemical vapor disposition SiO.sub.2 with K of &gt;3. Some organic and inorganic polymers have dielectric constants in the range of about 2.2 to 3.5, however, these have the problems of low thermal stability, poor mechanical properties including low glass transition temperature, sample outgassing, and long term reliability questions.
One solution is the use of nanoporous silica, which can have dielectric constants in the range of about 1 to 3. Nanoporous silica is particularly attractive due to the ability to carefully control its pore size and pore distribution, and because it employs similar precursors such as tetraethoxysilane (TEOS), as is presently used for spin-on glass (SOG's), and CVD SiO.sub.2. In addition to having low dielectric constants, nanoporous silica offers other advantages for microelectronics, including thermal stability up to 900.degree. C.; small pore size (&lt;&lt;microelectronics features); use of materials, namely silica and its precursors, that are widely used in the semiconductor industry; the ability to tune dielectric constant over a wide range; and deposition using similar tools as employed for conventional spin-on glass processing. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
A key parameter controlling property of importance for nanoporous silica dielectrics is porosity, the inverse of density. Higher porosity materials not only lead to a lower dielectric constant than dense materials, but they also allow additional components and processing steps to be introduced. As density decreases, dielectric constant and mechanical strength decrease but the pore size increases. Important issues relating to porous materials include pore size; the strength decrease associated with porosity; and the role of surface chemistry on dielectric constant, loss and environmental stability.
Nanoporous silica films are typically produced on substrates by methods such as dip-coating or spin-coating. One known process of silica deposition is a pre-mix method whereby a pre-mixed composition containing a silica precursor composition, including water and/or a base, and an organic solvent is deposited on a spinning substrate to thereby uniformly coat the substrate. One disadvantage of this pre-mix method is that the solvent and the silica precursor composition begin to react with each other immediately upon contact. Once this reaction starts, it cannot be stopped. The liquid properties of the mixture, such as viscosity, keep changing as the reaction continues. These property changes can result in clumping, etc, which contributes to the formation of a non-uniform film on the substrate. In another known process for silica deposition, a silica precursor composition is deposited onto a substrate and post-mixed with water and/or a base catalyst in vapor form. Such a method can lead to uniformity problems. Clearly, a method is needed for coating a nanoporous silica film onto a substrate which would minimize thickness variance and maximize film uniformity.
The present invention offers a solution to this problem. It has been unexpectedly found that the deposition of a combined composition stream of an alkoxysilane composition plus water or a base catalyst or both water and a base catalyst onto a surface of a substrate immediately after their combining results in a more uniform film. Alternatively, the alkoxysilane composition and optional base containing catalyst composition can form a combined composition stream which is exposed to water, such as water vapor, to produce a film. Using the combined stream method, the components of the combined composition stream first contact each other in the space above the substrate such that the combined stream is unbounded at the point of confluence of the individual streams. This minimizes reaction time between the components prior to deposition, which allows for better control of film properties such as film thickness, pore size, dielectric constant, etc. After exposure to water, the combined composition stream of this invention forms a gel which is then cured, aged, or dried. Using this process, a nanoporous silica film with a more uniform density and film thickness is formed onto a substrate.